After this move Black gains a clear advantage. Nakamura probably realized by now that he had no advantage and that it would be nearly impossible to play for a win. It was better to advance the kingside with 31.h4 or g4 or to simply play something like 31.Kg2.
28... d8c8  29. g2g3  29... a3a5 The queen is forced back. 30. b5f1  30... g8g7 